In general, a compressor is a mechanical apparatus receiving power from a power generation apparatus such as an electric motor, a turbine or the like, and compressing the air, refrigerant or various operation gases to raise a pressure. The compressor has been widely used for electric home appliances such as refrigerators and air conditioners, and application thereof has been expanded to the whole industry.
The compressors are roughly classified into a reciprocating compressor, wherein a compression space to/from which an operation gas is sucked and discharged is defined between a piston and a cylinder, and the piston linearly reciprocates in the cylinder to compress refrigerant, a rotary compressor, wherein a compression space to/from which an operation gas is sucked and discharged is defined between an eccentrically-rotating roller and a cylinder, and the roller eccentrically rotates along an inside wall of the cylinder to compress refrigerant, and a scroll compressor, wherein a compression space to/from which an operation gas is sucked and discharged is defined between an orbiting scroll and a fixed scroll, and the orbiting scroll rotates along the fixed scroll to compress refrigerant.
Recently, among the reciprocating compressors, a linear compressor has been actively developed because it improves compression efficiency and provides simple construction by removing a mechanical loss caused by motion conversion by directly connecting a piston to a linearly-reciprocating driving motor.
FIG. 1 is a side view illustrating a conventional linear compressor. The conventional linear compressor is installed such that a structure composed of a frame 2, a cylinder 3, a piston 4, a suction valve 6, a discharge valve 7, a linear motor 10, a motor cover 18, a supporter 19, a rear cover 20, main springs S1 and S2 and a suction muffler assembly 21 is elastically supported inside a shell 1.
The cylinder 3 is fixedly fitted into the frame 2, the discharge valve 7 is installed to block one end of the cylinder 3, the piston 4 is inserted into the cylinder 3, and the thin suction valve 6 is installed to open and close a suction hole 5 of the piston 4.
The linear motor 10 is installed such that a gap is maintained between an inner stator 12 and an outer stator 14 and a magnet frame 16 can linearly reciprocate therein. The magnet frame 16 is connected to the piston 4 by a piston fixing portion 16c, and linearly reciprocates due to a mutual electromagnetic force between the inner stator 12 and the outer stator 14 and the magnet frame 16 to operate the piston 4.
The motor cover 18 supports the outer stator 14 in an axial direction and is bolt-fixed to the frame 2 so as to fix the outer stator 14, and the rear cover 20 is coupled to the motor cover 18. The supporter 19 connected to the other end of the piston 4 is installed between the motor cover 18 and the rear cover 20 to be elastically supported by the main springs S1 and S2 in an axial direction, and the suction muffler assembly 21 which allows suction of refrigerant is also fastened with the supporter 19.
Here, the main springs S1 and S2 include four front springs S1 and four rear springs S2 in up-down and left-right symmetric positions around the supporter 19. When the linear motor 10 operates, the front springs S1 and the rear springs S2 move in opposite directions to buffer the shock of the piston 4 and the supporter 19. Moreover, refrigerant existing on the side of a compression space P serves as a kind of gas spring to buffer the shock of the piston 4 and the supporter 19.
Accordingly, when the linear motor 10 operates, the piston 4 and the suction muffler assembly 21 connected thereto linearly reciprocate, and the operations of the suction valve 6 and the discharge valve 7 are automatically controlled with variations of a pressure of the compression space P, so that the refrigerant is sucked into the compression space P via a suction tube (not shown), the suction muffler assembly 21 and the suction hole 5 of the piston 4, compressed therein, and discharged to the outside through a discharge cap 8, a loop pipe 9 and a discharge tube (not shown) on the shell side.
The linear motor 10 of the linear compressor includes the inner stator 12, the outer stator 14, and the magnet frame 16 around the frame 2 as shown in FIG. 1. The inner stator 12 is constructed such that laminations are stacked in a circumferential direction, and the outer stator 14 is constructed such that core blocks 14b are installed on a coil winding body 14a at certain intervals in a circumferential direction.
FIG. 2 is a perspective view illustrating a conventional magnet frame. The magnet frame 16 includes a cylindrical frame main body 16a positioned between the inner stator 12 and the outer stator 14 of the linear motor 10, magnets 16b fixed to some outer portions of the frame main body 16a, and a piston fixing portion 16c extended to the inside so that the piston 4 can be fixed to one end of the frame main body 16a. Holes 16d are formed on one side of the magnets 16b. 
Here, the magnets 16b are formed on the frame main body 16a at certain intervals in a circumferential direction. Preferably, eight magnets 16b are coupled to the outside of the frame main body 16a at regular intervals.
In the conventional linear compressor, the magnet linearly reciprocates between the inner stator and the outer stator due to a mutual electromagnetic force. However, it is difficult to employ a cylindrical magnet because of a high price of the magnet. Even if several bar-shaped magnets are fixed to form a magnet frame, the unit costs and overall costs of production still increase.
Moreover, in the conventional linear compressor, the linear motor varies a stroke to modulate a cooling capacity according to a load. To this end, a complicated control unit is provided, which is accompanied with design limitations on sizes of peripheral components. Further, a complicated control method is required, which increases the costs of production and complicates a manufacturing process. Furthermore, much power is consumed for controlling, which degrades efficiency of the whole compressor.